As an example of such switch devices of the prior art, the structure shown in FIGS. 1 through 3 is known. Specifically, the switch device shown in perspective view in FIG. 1 includes overlay-finished display board 1 provided with a number of key-tops 2, switch unit 3, double-sided adhesive tape 4 used to adhere the display board 1 to the switch unit 3, metal support board 5, made of iron or the like, and double-sided adhesive tape 6 used to adhere the switch unit 3 and the support board 5 one to another. These structural members are stacked in such an order as shown in the drawing, and the double-sided adhesive tape 4 is formed with square-shaped openings 7 at positions corresponding to those of the key-tops 2 of the display board 1.
The switch unit 3 is structured as shown in FIGS. 2 and 3, and comprises a flexible upper electrode 10 having on the under surface of an insulating film 8 contact portions 9 at positions corresponding to those of the key-tops 2 of the display board 1, an electrically insulating spacer 12 made of paper, synthetic resin film and the like and having thru-holes 11 at positions corresponding to those of the key-tops 2, and a lower electrode 15 having on the upper surface of an insulating film 13 contact portions 14 at positions correponding to those of the key-tops 2, these elements being piled up in that order. Accordingly, each contact portion 9 of the upper electrode 10 and corresponding contact portion 14 of the lower electrode 15 are facing one another within the thru-hold 11 of the spacer 12. Incidentally, each contact portion 9 of the upper electrode 10 and each contact portion 14 of the lower electrode 15 are connected, as shown in FIG. 1, through flexible cables 16, 17 to a control section (not shown).
FIG. 2 illsutrates the state of the switch unit 3 where the key-top 2 of the display board 1 is not depressed, so that the contact portions 9, 14 are spaced apart from one another by the spacer 12 interposed between the upper electrode 10 and the lower electrode 15 and, thus, the switch is in the off state.
If any key-top 2 of the display board 1 is pushed by the finger, in response thereto the upper electrode 10 is locally depressed as shown in FIG. 3 and the contact portion 9 of the upper electrode 10 comes into contact with the contact portion 14 of the lower electrode 15 thereby resulting in the switch-on state. As the finger is lifted from the key-top 2 of the display board 1, owing to the recovery force of the upper electrode 10, the contact portion 9 separates from the contact portion 14 and the switch-off state is restored.
According to the conventional switch device of the foregoing construction, because the display board 1, the switch unit 3 and support board 5 mutually differring in the rate of thermal expansion are overlaid, these members exhibit different rates of expansion/contraction in response to variation in temperature. Specifically, since the switch unit 3 and the support board 5 having significantly different rates of thermal expansion are bonded together in a wholescale way by the double-sided adhesive tape 6, as the switch unit 3 expands or contracts in response to variation in temperature, camber or wrinkles appear on the switch unit 3 and a positional slippage would occur between the contact portion 9 of the upper electrode 10 and the contact portion 14 of the lower electrode 15, thus, making the performance of the contacts inferior.